1. Field of the Invention
The present invention relates to a probe assembly for use in an infrared ear thermometer, and particularly to a probe assembly which fast adapts to a dynamic enviroment where temperature may vary greatly.
2. Description of Related Art
Infrared ear thermometers have been extensively used to measure human body temperatures. When using the infrared ear thermometer, the probe is positioned into the ear canal; the thermal radiation emitted by the eardrum and nearby tissues is directed to an infrared sensor through a waveguide; a contact temperature sensor tracks the temperature of the infrared sensor; an electronic circuit processes the signals of the infrared detector and the contact temperature sensor according to Stefan Boltzman""s law; and then a calculated human body temperature is displayed on a liquid crystal display.
Nowadays, most infrared ear thermometers employ an infrared detector, which has a thermopile chip and a thermistor packed in a same metal can and mounted on the same base plate. The purpose of this kind of arrangement is to keep the thermistor tracking the cold junction temperature of the thermopile chip. Inasmuch as the thermal time constat of the thermistor is different from that of the thermopile chip, it is easy to introduce measurement erros if the thermometer is in a dynamic environment where temperature may vary greatly along with time.
U.S. Pat. No. 4,895,164 discloses a probe assembly with a waveguide and an infrared detector being held in an isothermal state at ambient temperature by a heat conducting block. The drawback of this approch is that a substantially large heat conducting block is required. In addition, an isothermal condition is not necessary in the design of a probe assembly. Since the surfaces of most waveguides are coated with a layer of gold, which has very high reflectivity and very low emissivity, even though a temperature difference exists between the infrared detector and the waveguide, the infrared detector assembly receives little thermal radiation emitted from the inner wall of the waveguide.
U.S. Pat. No. 6,152,595 discloses a probe assembly comprising a radiation sensor (thermopile), a waveguide and a thermal coupling arrangement. As we can understand according to Stefan-Boltzman""s law, a probe assembly is impossible to be made without a contact temperature sensor (thermistor) detecting the cold juction temperature of the thermopile chip. Without considering the difference of the thermal time constants between the thermopile sensor and the thermistor, measurement erros will inevitably occur when the thermometer is in a dynamic environment. Furthermore, the thermal coupling arrangement comprising fives parts will complicate the assembly process.
FIG. 1 shows prior art probe 10 of an infrared thermometer, which comprises waveguide 11, heat conducting block 12 and infrared (IR) detector assembly 13. FIG. 2 shows the schematic diagram of IR detector assembly 13, which comprises metal can 21, IR filter 22, thermopile chip 23, thermistor 24, base plate 25 and leads 26. The thermal radiation directed by and through waveguide 11 first passes through IR filter 22 and is then detected by thermopile chip 23, and the temperature of the cold junction of thermopile chip 23 is represented by the temperature of thermistor 24, wherein thermistor 24 and thermopile chip 23 are placed on base plate 25.
The infrared thermometer calculates the temperature Ts of a target object according to Stefan Boltzman""s Law, as shown in equation (1):
Ts(t)=(Q(t)/KC+Ttp4(t))1/4xe2x80x83xe2x80x83(1) 
wherein Ttp represents the cold junction temperature of thermopile chip 23, Q represents the response of thermopile chip 23 to a thermal radiation source, KC represents a calibration coefficient. However, the cold junction temfperature Ttp of thermopile chip 23 is obtained by monitoring the temperatue of thermistor 24. Therefore, the temperature Tsxe2x80x2 of the target object calculated by the infrared thermometer is shown in equation (2),
Tsxe2x80x2(t)=((Q(t)+xcex94Q(t))/KC+Tth4(t))1/4xe2x80x83xe2x80x83(2) 
wherein Tth represents the temperature measured by thermistor 24, Q represents the response of thermopile chip 23 to a thermal radiation source, xcex94Q represents the response of the thermopile chip 23 to waveguide 11 and xcex94Q can be expressed by equation (3),
xcex94Q=(Twg4(t)xe2x88x92Ttp4(t))xc3x97∈Wxc3x97CWxe2x80x83xe2x80x83(3) 
wherein ∈W represents the emissivity of waveguide 11, Cw represents a coupling factor. Measurement error xcex94Ts is shown in equation (4),
xcex94Ts=Tsxe2x80x2(t)xe2x88x92Ts(t)xe2x80x83xe2x80x83(4) 
=((Q(t)+xcex94Q(t))/KC+Tth4(t))1/4xe2x88x92(Q(t)/KC+Ttp4(t))1/4 
=((Q(t)+(Twg4(t)xe2x88x92Ttp4(t))xc3x97∈Wxc3x97CW)/KC+Tth4(t))1/4xe2x88x92(Q(t)/KC+Ttp4(t))1/4 
In other words, in a static environment where the ambient temperature is stable, Tth(t)=Ttp(t)=Twg(t), xcex94Q=0, and the measurement error xcex94Ts is equal to zero. On the contrary, in a dynamic environment where the ambient temperature varies Tth(t)xe2x89xa0Ttp(t)xe2x89xa0Twg(t), xcex94Qxe2x89xa00, and a measurement error will occur.
FIG. 3 shows an equivalent model of the probe shown in FIG. 1 responding to temperature, wherein Tamb(t) represents the ambient temperature of the environment, Rhs represents the thermal resistance of the heat-conducting block, Chs represents the thermal capacitance of the heat-conducting block, Twg(t) represents the temperature of the waveguide, Rth represents the thermal resistance of the thermistor, Cth represents the thermal capacitance of the thermistor, Tth(t) represents the temperature of the thermistor, Rtp represents the thermal resistance of the thermopile chip""s cold junction, Ctp represents the thermal capacitance of the thermopile chip""s cold junction, and Ttp(t) represents the cold junction temperature of the thermopile chip.
FIG. 4 shows the temperature measurement error of the probe shown in FIG. 1, wherein curve 41 represents the temperature variation Tamb(t) of an environment and appears as a step function; curve 42 represents the temperature variation Tth(t) of the thermistor, and curve 43 represents the temperature variation Ttp(t) of the cold junction of the thermopile chip. Since the thermal resistance of the thermistor is less than that of the thermopile chip, the thermal time constant xcfx84th(xcfx84th=Rthxc3x97Cth) of the thermistor is less than the thermal time constant xcfx84tp(xcfx84tp=Rtpxc3x97Ctp) of the cold junction of thermopile chip, and the response of the thermistor to temperature variation is faster than that of the thermopile chip to temperature variation. In other words, before the prior art system becomes stable, the thermometer will introduce measurement errors.
Therefore, how to make a thermistor acurrately track the cold junction temperature of a thermopile chip without time lag is an important issue.
The main object of the present invention is to provide a probe for use in an infrared thermometer, which can accurately measure the temperature of a thermal radiation source in a dynamic environment.
The second object of the present invention is to provide a probe for use in an infrared thermometer, which is easy to manufacture and has a smaller volume than the prior arts.
The third object of the present invention is to provide a probe for use in an infrared thermometer, which could shorten the unstable time period in a dynamic environment.
For obtaining the above objects, the present invention discloses a probe for use in an infrared thermometer. The probe takes advantage of the heat transfer theorem and makes the thermistor accurately and fast respond to the actual cold junction temperature of a thermopile chip in order to reduce measurement errors to an acceptable level in a dynamic environment.
In a first embodiment of the present invention, the probe for use in an infrared thermometer comprises a waveguide, a first heat sink, an IR sensor, a second heat sink and at least one first silicon film. The first heat sink is thermally coupled to the waveguide. The IR sensor keeps an air gap from the first heat sink. The second heat sink is thermally coupled to the IR sensor for reducing the temperature variations of the IR sensor. The first silicon film is situated between the first heat sink and the second heat sink for increasing thermal resistance.
In a second embodiment of the present invention, the probe for use in an infrared thermometer comprises a waveguide, a first heat sink, an IR sensor and a second heat sink. The first heat sink is thermally coupled to the waveguide. The IR sensor keeps a gap from the first heat sink. The IR sensor comprises a thermistor, thermopile and a base plate, and is characterized in that a substrate of less thermal conductivity is situated between the thermistor and the base plate. The second heat sink is thermally coupled to the IR sensor for reducing the temperature variations of the IR sensor.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.